This invention relates to providing adequate pump power for driving single-mode fiber amplifier structures.
Single-mode 1.5 micron amplifier structures, whether erbium-doped fiber (EDFA), or planar waveguide, are essential optical components used in telecomm, optical networking, and free-space communications applications. Optical amplification is provided via stimulated emission from activated excited Er3+ ions at the signal wavelength, typically C-band, between 1530 and 1560 nm. The Er3+ ions are excited by an optical xe2x80x9cpumpxe2x80x9d source at either 980 or 1480 nm, which is usually generated by a diode laser.
The most commonly used pump sources for the 980 and 1480-nm bands are edge-emitting semiconductor lasers. Single-mode edge-emitting laser diodes are useful pump sources; however, their output power is severely limited because optical damage of the output facets occurs power at densities of about 10 MW/cm2. As a consequence, single-mode pump sources are limited to about 350 mW of fiber-coupled output power so that high-power applications require multiple sources in each pump band and/or amplifier stages, which is complex and costly. Currently, the maximum rated fiber-coupled power for 980- and 1480-nm sources are 300 and 240 mW, respectively. The lifetime for telecomm components is measured in FIT, shorthand for failures-in-time. One FIT equals a single failure per 109 device hours and currently 980- and 1480-nm diode sources are estimated to have less than  less than 50 FIT. However at a unit cost of $1,200 and $4,500 per diode this failure rate leaves much to be desired.
Single-mode laser power is also limited by the narrowness of the gain region of edge-emitting laser diodes. Unfortunately, attempting to increase output power by increasing the width of the gain region causes higher order modes to begin to oscillate, thus reducing the brightness of the source. This effect is noticed when the spotsize within the semiconductor resonator is larger than the single-mode spotsize, e.g.  greater than 2 xcexcm. Therefore while such xe2x80x9cmulti-modexe2x80x9d edge-emitting diode laser sources can output high optical powers e.g.,  greater than 2 W, they are not viable single-mode pump sources for EDFAs and other Er-doped amplifying structures.
The need for a reliable, high power, single-mode pump source is particularly evident in long haul fiber-optic submarine cable networks which utilize remote pump sources to pump submerged Er-doped amplifier structures. These submerged amplifiers are distributed over several thousands of kilometers of fiber, so it is essential that the pump source operate in a spectral region that is highly transparent in the host fiber. In these cases, lasers that operate in the 1480-nm pump band are the only practical remote pump sources, since 1480-nm is located within the low-loss transmission window of silica fibers. The spacing between amplifiers typically ranges between 30 to 100 km; therefore, several tens of amplifiers are required to span a typical length of long-haul fiber e.g., 2000 to 8000 km. Because remotely pumped amplifiers are pumped in serial, the power required to pump multiple amplifiers scales with the number of amplifiers and with the number of channels. In practice, more watts of pump power are required to pump repeaterless long-haul networks than can conveniently be provided by the current state-of-the-art.
In addition to high power, the stability of pump wavelength is an important factor in obtaining optimum performance of the amplifying structure. Present day devices require special conditioning and feedback to achieve xe2x80x9cgain-flatteningxe2x80x9d over the amplifier bandwidth. The most commonly employed means of stabilizing the output wavelength of semiconductor pump sources is via passive external feedback from a fiber Bragg grating (c.f., U.S. Pat. No. 6,044,093 and references therein). It would therefore be extremely advantageous to be able to economically generate several watts of frequency stable optical power in either or both the 980 and 1480-nm pump bands used in these EDFA applications without the need for wavelength stabilization measures.
We have recognized that the shortcomings of the prior art can be overcome by converting the mode and wavelength of a conventional, multi-mode wide-stripe laser diode to a stable, single-mode output that falls within the spectral region required for pumping EDFA amplifying structures. A diode laser is chosen that operates at a wavelength that corresponds with the 4I9/2xe2x86x924F5/2 absorption band of Nd3+ ions impregnated into a laser host crystal. The host crystal provides an appropriate environment to shift the known xe2x80x9c4fxe2x80x9d laser transitions to the absorption bands required by single-mode amplifying structures doped with Er3+ and/or Yb3+ that are to be pumped. The energy absorbed by the Nd3+ ions in the host crystal is re-radiated into the 4F3/2xe2x86x924I9/2, 4F3/2xe2x86x924I11/2, and 4F3/2xe2x86x924I13/2 transitions which release photon energy that can be utilized by amplifying structures doped with Er3+ and/or Yb3+. The spatial mode of the multimode laser diode is converted to single-mode by enclosing the Nd3+-doped host laser crystal within a laser cavity that has a fundamental mode size large enough to encircle the spatial extent of the beam emanating from the laser diode. The resulting laser runs, or is forced to run, in a single spatial mode, which is appropriate for coupling into single-mode fiber and/or single-mode amplifying structures.